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Near-infrared spectral observations and interpretations for S-asteroids 138 Tolosa, 306 Unitas, 346 Hermentaria, and 480 Hansa |
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| Authors: | Paul S Hardersen Michael J Gaffey Edward A Cloutis Paul A Abell Vishnu Reddy |
| Institution: | a Department of Space Studies, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA b Department of Geography, Room 5L13, University of Winnipeg, Manitoba, Canada c Planetary Astronomy Group, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Mail Code SR, Houston, TX 77058, USA |
| Abstract: | Near-infrared (∼0.7 to ∼2.5 μm) spectra of S-asteroids 138 Tolosa, 306 Unitas, 346 Hermentaria, and 480 Hansa suggest the presence of variable amounts of orthopyroxene ± clinopyroxene ± olivine ± plagioclase feldspar on the surfaces of these asteroids. The spectra of these asteroids were compared to laboratory mineral mixtures of orthopyroxene, clinopyroxene, and olivine Singer, R.B., 1981. J. Geophys. Res. 86 (B9), 7967-7982; Cloutis, E.A., 1985. Master's thesis]. The band parameters (band centers, band areas) were quantified and temperature-corrected Moroz et al., 2000. Icarus 147, 79-93; Gaffey et al., 2002. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. The University of Arizona Press, Tucson, pp. 183-204]. Each S-asteroid in this paper exhibits an overall spectral shape with band parameters that are inconsistent with ordinary chondrite near-infrared spectra and their inferred mineral abundances and/or pyroxene chemistries. 138 Tolosa displays a complex spectrum with a broad ∼1 μm absorption feature that displays a double Band I minimum, a well-defined absorption at ∼1.3 μm, and a broad, but weak absorption in the ∼2 μm region. Although different interpretations exist, the Tolosa spectrum is most consistent with a ∼60/40 mixture of Type B clinopyroxene and orthopyroxene. Spectra of 306 Unitas suggest a surface with variable amounts of low-Ca pyroxene and olivine. Unitas is located in the S-(IV) and S-(VI) subtype regions in Gaffey et al. 1993. Icarus 106, 573-602]. 346 Hermentaria exhibits a complex, broad Band I absorption feature and a weak Band II feature, which suggests a ∼50/50 mixture of clinopyroxene and orthopyroxene. Hermentaria is classified as an S-(III). Spectra of 480 Hansa suggest a dominant low-Ca pyroxene component with lesser amounts of olivine. Based on these characterizations, these four S-asteroids should not be considered as potential ordinary chondrite parent bodies. Furthermore, these results suggest that these S-asteroids experienced at least partial melting temperatures T?∼950 °C: Gaffey et al., 1993. Icarus 106, 573-602; Keil, K., 2000. Planet. Space Sci. 48, 887-903] during the formation epoch in the early Solar System. Continuing spectroscopic investigations will discern the relative abundance of chondritic and thermally-evolved objects among the S-type asteroids that have survived since the formation epoch ∼4.56 billion years ago. |
| Keywords: | Asteroids composition Mineralogy Spectroscopy Thermal histories |
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